WASHINGTON, USA (AFP) — A top aide to Mitt Romney yesterday accused President Barack Obama of going lower than a "world champion limbo dancer" with negative attack ads he said have demeaned the office of the US presidency.

Eric Fehrnstrom expressed revulsion at a recent television ad airing in key swing states and put together by a pro-Obama Super PAC which insinuates that Romney was partly to blame for the death of a woman whose husband lost his job at a steel mill taken over by Romney's private equity firm Bain Capital.

The ad has become a flashpoint in the increasingly nasty campaign leading up to the November 6 election, and while an outside group released it, Fehrnstrom laid the blame for the ad at the feet of the president's re-election team.

"They've gone from what started as petty distortions and untruths to unbelievable exaggerations that diminish the office of the president and insult the American people," Fehrnstrom told reporters.

"I don't think a world champion limbo dancer could get any lower than the Obama campaign right now," he said, adding that the Romney campaign would not hesitate to counter the negativity with material of their own.

"We're going to continue to respond forcefully to Obama's ugly distortions and lies," Fehrnstrom said, speaking at Romney's campaign headquarters in Boston.

The ad, by political group Priorities USA, has been savaged by critics, and independent fact-checkers have called it irresponsible.

It features Joe Soptic, who was laid off from a Missouri steel plant that was taken over by Bain in the 1990s, when Romney ran the company.

Soptic says in the spot that he lost his health insurance when the plant was shut down, in 2001, and that his wife contracted cancer and died "a short time after" that.

But independent monitor FactCheck.org was among many that pointed out Soptic's wife died in 2006, and that she had lost her own employer-sponsored coverage a year or two after her husband was laid off.

Romney left Bain in 1999 to run the Salt Lake City Olympics.

"It strains the facts to the breaking point to imply that this tragic death is Romney's doing," FactCheck.org said.

The Romney campaign released its own ad, called "America Deserves Better," on Friday.

"What does it say about a president's character when his campaign tries to use the tragedy of a woman's death for political gain?" the ad's voice-over begins.